When packaging articles, such as bottles or cans, into a carton or other suitable container, the articles are typically separated into discrete groups of articles and each group of articles is then placed into a carton. Frequently, an insert or other suitable type of partition is placed between the articles to prevent the articles from colliding with each other. During the packaging process, a stack of cartons is formed, a single carton is selected from the stack, and the single carton is delivered to a carton transport assembly which places the carton in a position to receive the group of articles. Similarly, a stack of inserts is formed, a single insert is selected from the stack, and the single insert is placed into position between the individual articles in the group.
The existing packaging machines vary greatly in how they form a stack of products, which generically covers either a stack of cartons or inserts, and how they select a single product from the stack. In broad terms, however, the packaging machines form a stack of products by aligning the products face-to-face with the sides of the products abutting against some type of side rails and with the bottoms of the products resting against some type of floor. At least one tab or other type of projection typically contacts the first product in the stack to prevent that first product from separating from the stack. To remove the first product, a set of vacuum cups are moved against the first product, a vacuum is generated in the cups to securely hold the cups to the product, and then the vacuum cups along with the first product are moved away from the stack. The tab prevents the other products from leaving when the first product is removed. The removal of a single product from the stack is called "picking" the product and the position of the product which provides the best opportunity for a pick is termed the "pick plane".
It is difficult, however, to pick only one product from the stack. Some factors influencing the ease with which a product may be picked include the position of the tab or projection, the amount of pressure in the vacuum cups, and the weight of the stack against the tab or projection. The difficulty in setting the values for theses factors is that after the setting has been adjusted for one factor the settings for the other two factors might also need adjusting. Thus, the factors cannot be adjusted independently of each other.
For instance, the tabs must be positioned far enough into the product so that the force from the stack will not push the products past the tab, yet not be too far into the product so that the vacuum cups cannot remove a product. If the force supplied by the stack is too small, the vacuum cups knock the products out of the pick plane when the vacuum cups swing over for the pick. Thus, the vacuum cups will be unable to remove a product with too small of a force supplied to the tabs. On the other hand, if the force of the stack is too large for the tabbing, the tabbing cannot contain the products in the stack and the products are pushed past the tabbing. Further, while the vacuum cups must have enough pressure to overcome the resistance provided by the tabbing, the product may be torn or deformed with a heavy tabbing and a large pressure.
A common approach in the industry is to select a moderate pressure, a moderate tabbing, and to vary the amount of force supplied by the stack. The force supplied by the stack is mainly either from a component of the stack's weight or is from an external device, such as a paddle, pushing the rear of the stack.
To generate a force at the tabbing from a component of the stack's weight, the stack is formed at a downward angle with the first product being at a location lower than the last product in the stack. With this arrangement, the angle of the stack and the weight of the products in the stack will then determine the amount of force supplied at the tabbing.
The previous systems which used the weight of the stack to apply a force at the tabs adjusted the amount of the force by varying the number of products in the stack. With one system, a conveyor belt holding a reserve of products would be activated to drop more products in the stack when a photocell detected that the stack has been reduced to a certain thickness. When the stack is at that certain thickness, the stack is suppose to generate the desired force at the tabs.
A difficulty with this system is that the weight of the stack changes when the product is replaced with a different product. With a product having a different size or weight, the photocell would no longer be in the proper position and the weight of the stack would be too large or too small for the particular tabbing and for the particular pressure in the vacuum cups. While the position of the photocell could conceivably be adjusted, this would only further complicate matters by requiring an operator to precisely position the photocell.
The adjustment of the photocell would present further difficulties as well. For instance, the mechanism for dropping the products into the stack requires a certain distance between the reservoir of products and the rear of the stack in order for the products to fall into alignment with the other products in the stack. If the position of the photocell were to change, the distance between the rear of the stack and the reservoir would change which might prevent the products from falling into alignment with the other products.
With another type of system, the number of products in the stack is roughly controlled by a limit switch positioned against the first product in the stack. The function of the limit switch in this type of system is basically to inform a controller whether the first product in the stack is in the proper position for a pick. The limit switch has a spring-biased plunger which is depressed when the first product is in position. When the limit switch is not depressed, the system will increase the number of products in the stack or advance the stack closer to the tabs in order to move the first product against the limit switch.
A problem with the limit switch system is that it can only indicate whether or not the first product is in the proper position. The supply of products into the stack is simply an on/off control resulting in a variable amount of force being supplied to the tabbing. In other words, the limit switch just ensures that the force is at least above a certain level and does not prevent the force from becoming too large for a particular tab setting. The limit switch system is therefore not an ideal way for controlling the amount of force at the tabbing.
Another problem with the limit switch system is that the limit switch must be precisely located relative to the products in the stack. If the limit switch is too far away from the first product in the stack, the limit switch will cause the system to add too many products thereby producing a larger than desired force at the tabbing. When the limit switch is positioned too close to the first product, the force at the tabbing will be insufficient and the products will be knocked out of position by the vacuum cups. Further, the limit switch is a mechanical switch which has a limited life-time which will need to be replaced periodically.
A need therefore exists in the industry for a product delivery system that can consistently and reliably deliver a single product from a stack of products. A need also exists for a system that can accurately control the force supplied from a stack of products.